(1) Field of the Invention
The present invention is directed generally towards piezoelectric transducers, and in particular, to a piezoelectric transducer that uses a crystalline phase transformation to achieve improved elongation.
(2) Description of the Prior Art
Most transduction devices are based on strain produced in a piezoelectric or magnetostrictive material. These materials can produce relatively large strain in a linear region, but in order to develop high strain they must be driven by a very high electric or magnetic field. There are several other classes of materials utilizing high strain associated with martensitic phase transition (namely, shape memory alloys). However, high strain in shape memory alloys (SMA) is thermally activated, and transduction devices based on these materials have frequency band limitations. For this reason, piezoelectric and magnetostrictive materials are most often used in transducer applications.
The current most promising class of transducer materials are relaxor-ferroelectric piezoelectric single crystals. These materials are single crystals of piezoelectric materials (for example, lead zinc niobate-lead titanate, known hereinafter as PZN-PT). These materials have been shown to deliver extraordinarily high strain when an external electric field is applied as compared to conventional polycrystalline piezoceramic.
In some special compositions (for example ternary lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT)), the material will undergo a phase transformation accompanied by a very sharp hysteretic strain and a dramatic change in stiffness when subjected to external stress. This phase transformation can be invoked repeatedly at variable rates to induce large strains in the single crystal element. Known compositions exhibiting this type of phase change behavior include (1−x)PZN-xPT where 0.04<x<0.11. The composition where x was 0.06 has been tested at multiple temperatures and applied direct current (DC) bias fields. Under these conditions, it has been shown to exhibit a phase transformation.
It is known to combine single crystal piezoelectric materials with mechanical stress inducing means. This is typically performed in order to avoid putting the piezoelectric material in tensile stress because of the fragility of ceramic materials in tension. In a prior art mechanically induced stress application, the stress is calculated to be that which is optimal for insuring piezoelectric material life in the operating conditions of the application. These operating conditions can include varying environmental temperatures and pressures.
It is also known to include an electrically controllable stress element in combination with a piezoelectric single crystal material. This element can be either a piezoelectric (voltage driven) element or a magnetostrictive effect (MS) element. These hybrid magnetostrictive-piezoelectric transducer systems are known to work effectively in a linear region.
In both mechanical and electrical stress generation means, applications avoid utilizing stress near the phase transition stress level. This is because the piezoelectric material shows a non-linear response at this level that may be triggered by environmental temperature or pressure.